1. Cross Reference
This application claims the benefit of U.S. provisional application Ser. No. 60/825,941 filed on Sep. 18, 2006, the subject matter of which is incorporated herein by reference.
2. Field of the Invention
The invention relates to time synchronization in wireless communication systems, and, more particularly, to techniques of time synchronization for low carrier-to-interference ratio signals.
FIG. 1 shows a wireless communications receiver. The receiver comprises a RF front end 102, a synchronization circuit 104, a channel estimator 106, an equalizer (EQ) 108, and an outer receiver 110. The RF front end 102 receives and processes radio frequency signals from a channel into baseband signals. Synchronization circuit 104 synchronizes the received packets by determining the frame and slot boundary of each packet. The synchronized data is sent to equalizers 108 and outer receiver 110 for further processing such as decoding, decompressing, and others. Synchronization circuit 104 is essential for any communications system employing coherent detection.
Typically, a transmitted signal includes additional signal data for synchronizing communication among receivers and transmitters. The additional signal data may comprise a preamble, training sequence, synchronization burst, synchronization byte, and others. The synchronization burst is known by the receiver in advance, the timing of the sequence when it is received is, however, unknown. In most communication systems, synchronization is accomplished by correlating a received sequence with a local sequence. A correlator computes a correlation result indicating how much the received sequence and the local sequence are related.
Correlation results are highly sensitive to carrier-to-interference ratio (C/I). For example, correlation results break down at a C/I ratio lower than −8 dB. Thus, a robust time synchronization scheme, capable of sustaining a poor C/I ratio, is desirable.